The thin-film transistor which is an example field-effect transistor uses an amorphous silicon film and/or a polycrystalline silicon film formed on an insulation substrate including glass or the like, as an element region. In recent years, the thin-film transistor has been actively developed, as a pixel switch of the active-matrix display device. The thin-film transistor includes a gate, a drain, and a source, and passes a current between the source and the drain according to a voltage applied to the gate. When the thin-film transistor operates in a saturation region, a drain current Ids is provided according to the following transistor-characteristic expression.Ids=(½)μ(W/L)Cox(Vgs−Vth)2
Here, Vgs represents the gate voltage with reference to the source, Vth represents a threshold voltage, Cox represents a gate capacitor, W represents a channel width, L represents a channel length, and μ represents the mobility of a semiconductor film. As is clear from the transistor-characteristic expression, when the gate voltage Vgs of the thin-film transistor exceeds the threshold voltage Vth, the drain current Ids is passed.
Several thin-film transistors are connected so that a transistor circuit having a predetermined function is formed. In general, the transistor circuit includes a plurality of thin-film transistors formed on a substrate and wiring adapted to connect the gate, source, and/or drain of each of the transistors, so as to perform a predetermined operation. A pixel circuit is a typical example of the above-described transistor circuit. The pixel circuit is formed at each of intersections of scan lines in rows and signal lines in columns so that the entire pixel circuits form the active-matrix display device. Upon being selected by the scan line, the pixel circuit operates, so as to sample a video signal from the signal line and drive a load element such as an organic EL light-emission element. The above-described active matrix-organic EL-display device including the thin-film transistor, as an active element, is disclosed in Japanese Unexamined Patent Application Publication No. 8-234683, for example.
In the saturation region, when the gate voltage exceeds the threshold voltage, the thin-film transistor is turned on and the drain current is passed, as is clear from the above-described transistor-characteristic expression. On the other hand, when the gate voltage becomes lower than the threshold voltage, the thin-film transistor is cut off. However, the threshold voltage Vth of the thin-film transistor is not necessarily constant but varied with time. Due to the variation in the threshold voltage, the cut-off operation is disturbed, which causes the transistor circuit to malfunction. Further, as is clear from the above-described transistor-characteristic expression, the drain current varies as the threshold value varies, even though the gate voltage is maintained constant. In the case of a pixel circuit configured to drive a light-emission element by a current, the drain current is varied due to the variation in the threshold voltage, so that the brightness of the light-emission element is deteriorated.